/* Copyright (c) 2017, Google Inc.
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */

#ifndef OPENSSL_HEADER_SSL_SPAN_H
#define OPENSSL_HEADER_SSL_SPAN_H

#include <openssl/base.h>

#if !defined(BORINGSSL_NO_CXX)

extern "C++" {

    #include <algorithm>
    #include <cstdlib>
    #include <type_traits>

BSSL_NAMESPACE_BEGIN

template <typename T> class Span;

namespace internal
{
    template <typename T> class SpanBase
    {
        // Put comparison operator implementations into a base class with const T, so
        // they can be used with any type that implicitly converts into a Span.
        static_assert(std::is_const<T>::value, "Span<T> must be derived from SpanBase<const T>");

        friend bool operator==(Span<T> lhs, Span<T> rhs)
        {
            // MSVC issues warning C4996 because std::equal is unsafe. The pragma to
            // suppress the warning mysteriously has no effect, hence this
            // implementation. See
            // https://msdn.microsoft.com/en-us/library/aa985974.aspx.
            if (lhs.size() != rhs.size()) {
                return false;
            }
            for (T *l = lhs.begin(), *r = rhs.begin(); l != lhs.end() && r != rhs.end(); ++l, ++r) {
                if (*l != *r) {
                    return false;
                }
            }
            return true;
        }

        friend bool operator!=(Span<T> lhs, Span<T> rhs)
        {
            return !(lhs == rhs);
        }
    };
} // namespace internal

// A Span<T> is a non-owning reference to a contiguous array of objects of type
// |T|. Conceptually, a Span is a simple a pointer to |T| and a count of
// elements accessible via that pointer. The elements referenced by the Span can
// be mutated if |T| is mutable.
//
// A Span can be constructed from container types implementing |data()| and
// |size()| methods. If |T| is constant, construction from a container type is
// implicit. This allows writing methods that accept data from some unspecified
// container type:
//
// // Foo views data referenced by v.
// void Foo(bssl::Span<const uint8_t> v) { ... }
//
// std::vector<uint8_t> vec;
// Foo(vec);
//
// For mutable Spans, conversion is explicit:
//
// // FooMutate mutates data referenced by v.
// void FooMutate(bssl::Span<uint8_t> v) { ... }
//
// FooMutate(bssl::Span<uint8_t>(vec));
//
// You can also use the |MakeSpan| and |MakeConstSpan| factory methods to
// construct Spans in order to deduce the type of the Span automatically.
//
// FooMutate(bssl::MakeSpan(vec));
//
// Note that Spans have value type sematics. They are cheap to construct and
// copy, and should be passed by value whenever a method would otherwise accept
// a reference or pointer to a container or array.
template <typename T> class Span : private internal::SpanBase<const T>
{
  private:
    // Heuristically test whether C is a container type that can be converted into
    // a Span by checking for data() and size() member functions.
    //
    // TODO(davidben): Switch everything to std::enable_if_t when we remove
    // support for MSVC 2015. Although we could write our own enable_if_t and MSVC
    // 2015 has std::enable_if_t anyway, MSVC 2015's SFINAE implementation is
    // problematic and does not work below unless we write the ::type at use.
    template <typename C>
    using EnableIfContainer = std::enable_if<std::is_convertible<decltype(std::declval<C>().data()), T*>::value &&
                                             std::is_integral<decltype(std::declval<C>().size())>::value>;

    static const size_t npos = static_cast<size_t>(-1);

  public:
    constexpr Span() : Span(nullptr, 0)
    {
    }
    constexpr Span(T* ptr, size_t len) : data_(ptr), size_(len)
    {
    }

    template <size_t N> constexpr Span(T (&array)[N]) : Span(array, N)
    {
    }

    template <typename C, typename = typename EnableIfContainer<C>::type,
              typename = typename std::enable_if<std::is_const<T>::value, C>::type>
    Span(const C& container) : data_(container.data()), size_(container.size())
    {
    }

    template <typename C, typename = typename EnableIfContainer<C>::type,
              typename = typename std::enable_if<!std::is_const<T>::value, C>::type>
    explicit Span(C& container) : data_(container.data()), size_(container.size())
    {
    }

    T* data() const
    {
        return data_;
    }
    size_t size() const
    {
        return size_;
    }
    bool empty() const
    {
        return size_ == 0;
    }

    T* begin() const
    {
        return data_;
    }
    const T* cbegin() const
    {
        return data_;
    }
    T* end() const
    {
        return data_ + size_;
    }
    const T* cend() const
    {
        return end();
    }

    T& front() const
    {
        if (size_ == 0) {
            abort();
        }
        return data_[0];
    }
    T& back() const
    {
        if (size_ == 0) {
            abort();
        }
        return data_[size_ - 1];
    }

    T& operator[](size_t i) const
    {
        if (i >= size_) {
            abort();
        }
        return data_[i];
    }
    T& at(size_t i) const
    {
        return (*this)[i];
    }

    Span subspan(size_t pos = 0, size_t len = npos) const
    {
        if (pos > size_) {
            abort(); // absl::Span throws an exception here.
        }
        return Span(data_ + pos, std::min(size_ - pos, len));
    }

  private:
    T* data_;
    size_t size_;
};

template <typename T> const size_t Span<T>::npos;

template <typename T> Span<T> MakeSpan(T* ptr, size_t size)
{
    return Span<T>(ptr, size);
}

template <typename C> auto MakeSpan(C& c) -> decltype(MakeSpan(c.data(), c.size()))
{
    return MakeSpan(c.data(), c.size());
}

template <typename T> Span<const T> MakeConstSpan(T* ptr, size_t size)
{
    return Span<const T>(ptr, size);
}

template <typename C> auto MakeConstSpan(const C& c) -> decltype(MakeConstSpan(c.data(), c.size()))
{
    return MakeConstSpan(c.data(), c.size());
}

BSSL_NAMESPACE_END

} // extern C++

#endif // !defined(BORINGSSL_NO_CXX)

#endif // OPENSSL_HEADER_SSL_SPAN_H
